Group III–V Nitride compound semiconductor devices include both light emitting devices and electronic devices. Light emitting devices may be tailored by film composition to emit light in the range continuously from amber to green, blue and finally ultraviolet. By proper combination with other color light emitting devices or adding phosphors to these devices, it is also possible to generate “white light”. The emission pattern of such device may be incoherent, and hence be termed as a “light emitting diode” (LED), or else the emission may be coherent, in which case the device is termed a “laser diode” (LD). The electronic devices may also include high electron mobility transistors (HEMT), heterojunction bipolar transistors (HBT), Schottky, p-i-n and metal-semiconductor-metal (MSM) photodiodes and others.
Sapphire was among the first materials used to grow GaN thin films and to produce blue and green color LEDs. It is often still the material of choice because of its relatively low cost and availability in the commercial marketplace. The brightness of the LEDs made on sapphire is adequate because of the transparency of the sapphire substrate so that the light can be effectively emitted without too much blockage.
Unfortunately, a GaN film on sapphire has a high defect density due to poor lattice mismatching (>17%). An attempted solution for the poor mismatch has been to grow a low temperature buffer layer of AlN prior to the growth of GaN. The GaN layer is grown over the nuclei of AlN which is highly oriented with the c-axis up. Even though the GaN layer is technically polycrystalline it is still suitable to make ordinary LED devices. The typical dislocation density of GaN film on the sapphire with the buffer layer is on the order of 1011 per cm2, although there is evidence that the dislocation density can be reduced by growing a thicker layer of the GaN film due to grain growth and reduction of grain boundaries. The improvement is limited and it costs more to grow thicker films.
To make high performance devices, sapphire suffers from not being such a good thermal conductor as compared to GaN, AlN, SiC and even Si, for example. As a result, it is difficult to produce high brightness LEDs that require higher current injection and thus more heat generated. Moreover, the bonding of GaN onto sapphire is very strong and difficult to remove, and sapphire is an insulator. Both of these increase the fabrication steps to produce the LEDs. The device size is bigger and the number of diodes produced per unit area is less because both electric leads are on the same side of the diodes.
To make laser diodes (LDs) on sapphire, one suffers the same problems of high defect density and poor thermal conductivity that limit the current density and thus the power output of the laser. Moreover, since the GaN film is composed of polycrystalline grains, it is difficult to produce a smooth surface face for resonating cavities. As a result, the mode structure of the laser is poor.
Another method has been developed to use epitaxial lateral overgrowth (ELOG) to create small regions with a relatively large GaN crystal grain size and low defect density. LDs made from these selected low defect regions indeed show improved performance. Unfortunately, the overall process is complicated and expensive, and the LD yield is very low.
An alternative approach is to use SiC as substrate to grow a GaN thin film. The lattice match of SiC to GaN is much improved (<3.5%) as compared to that of sapphire. The theoretical defect density is also greatly reduced, on the order of 109 per cm2. Perhaps most important of all is that with low lattice mismatching, the GaN film grown on the SiC substrate can be considered as single crystal film as compared to the polycrystalline film on sapphire.
However, growing high quality GaN thin films on SiC wafers does present a number of problems. First, SiC wafers are expensive because the growth of the SiC crystal is difficult. It is produced by a physical vapor transport method at a very high temperature (greater than 2200° C.) using specially designed vacuum sealed reactors. Second, the cutting and polishing processes are also expensive due to the high hardness of SiC being close to that of diamond. Third, the smaller thermal expansion coefficient of SiC (4.2×10−6/° C.) relative to GaN (5.6×10−6/° C.) is also problematic since it may force the GaN film under tension and cause cracks during cooling after growth.
A special multi-AlGaN layer film may be grown first on the SiC wafer before the final growth of GaN film to reduce such cracking. The same layers also serve the purpose to minimize the band gap offset between SiC and GaN. With such offset minimized, it is possible to use the beneficial feature of electrical conductivity of the SiC substrate to build GaN LEDs with the conventional design. This greatly reduces the size of the LEDs and the yield per unit area is much higher than that made from sapphire. Higher yield compensates for the high cost of substrate material. SiC also has the advantage of high thermal conductivity. This in combination with low defect density should make the LEDs and LDs perform better using the SiC substrate.
Indeed, the intrinsic quantum efficiency of GaN LEDs made on SiC is better than that on sapphire. However, the overall external brightness of GaN LEDs on SiC is worse. This is because that SiC is not as transparent to the emitted light of GaN so that a significant amount of light is blocked. This is particularly true for the UV LEDs. On the other hand, the performance GaN LDs on SiC is much better since good cleaved surfaces are achievable. The beam quality of the laser has much simpler mode structure and more suitable for DVD type of application. The high thermal conductivity of SiC substrate also means that higher current can be applied across the LDs and thus increase the power output.
The result of GaN films on sapphire and SiC points to a common conclusion that to further improve the performance of LEDs and LDs, there is a need to be able to grow low defect density GaN films. In other words, the substrate should have a closely matched lattice constant to that of GaN. Moreover, the substrate should also be transparent and have good electrical and thermal conductivity. At present time, the only substrate that can satisfy all these requirements is single crystal GaN substrate. Unfortunately, the technology to produce such a single crystal GaN substrate may not yet be sufficient.
UNIPRESS of Poland has developed high pressure process to produce true single crystal GaN in thin flake morphology up to a centimeter in size, but this may not be a commercially viable mass production process. There are others, such as ATMI, Lincoln Laboratory of the USA and Samsong of Korea who have successfully produced thick free standing GaN wafers of a few centimeters in size. Unfortunately, mismatched thermal expansion coefficients tend to bend and crack the wafer after growth. To free GaN from sapphire, a laser ablation technique has been used. The removed GaN wafer still needs to be polished to be useful.
Another material with a good potential may be the single crystal AlN substrate. Small single crystals have been produced by a physical vapor transport technique under similar high temperature condition as SiC. The growth process is still under the development stage, and high quality wafers of AlN may not be available for many years to come. Moreover, AlN is an insulator. So device fabrication will face the same constraints as those on sapphire.
Another alternative is to search for a surrogate substrate that has good lattice matching to GaN. After the growth of the GaN films on this substrate the surrogate substrate may be removed to obtain a free-standing single crystal GaN film. If the GaN film has adequate thickness, it will be strong enough and can then be used as substrate wafers for manufacture GaN LEDs and LDs. Sumitomo, for example, uses GaAs as a surrogate substrate in combination with ELOG technology and is able to produce two inch diameter free-standing GaN wafers. The GaAs substrate is removed by chemical etching after the growth of thick film of GaN. Since the GaN surface is very rough after growth, polishing is needed to produce smooth surface. The overall process is still complicated and the cost of the wafer is high. Sumitomo's free-standing GaN wafer is c-face (0001) oriented. Because of the large lattice misfit (>45%) between GaAs and GaN, Sumitomo's free-standing GaN wafer is polycrystalline.
In U.S. Pat. No. 5,625,202, Chai discloses a large class of compounds suitable as substrate materials for the growth of GaN and AlN single crystal films. Among the listed compounds, LiAlO2 (LAO) and LiGaO2 (LGO) show the best potential. This is because large size single crystals of both LAO and LGO can be produced by the standard Czochralski melt pulling technique. The technology to produce large diameter high quality single crystal substrates is ready now and the growth of GaN thin films has been demonstrated on both LAO and LGO substrates.
During the growth process, it is noticed that the compatibility of the chemicals to produce GaN films with an LGO substrate is very poor despite the fact that the two crystals have the best lattice matching and nearly identical crystal structure. The chemicals to grow the GaN film will attack the surface of LGO during growth. Moreover, even if GaN film is able to grow on the LGO substrate, the adherence of the GaN film is very poor so it will inevitably peel off after growth due to mismatch of the thermal expansion coefficients.
LAO has a very different crystal structure and crystal symmetry (tetragonal) from GaN (hexagonal). Nevertheless, the two dimensional (100) surface of LAO has nearly the same structure and lattice dimension as the m-face (1010) of GaN. The lattice mismatch along the a-axis direction of GaN is +1.45%. The lattice mismatch along c-axis direction of GaN is only −0.17%. The chemical compatibility of LAO to the growth chemicals of GaN is also much better. Perhaps most important of all is that LAO wafer can easily be removed after growth with simple acid etching. Utilizing such unique properties free-standing single crystal GaN wafers have been produced with a thickness in the range from 150 up to 500 μm using the HVPE (metal hydrite vapor transport epitaxial growth) method. The single crystal GaN wafer produced from the LAO substrate has the m-face orientation with the index of (1010). It is distinctly different from all the other free-standing GaN wafers available in the market since they all have a c-face orientation with the index of (0001). These wafers are disclosed in U.S. Pat. No. 6,648,966 and published U.S. application No. 2003/0183158, both assigned to the assignee of the present invention, and the entire contents of which are disclosed herein by reference.
The easy removal of the substrate with the simple acid etching is a desirable property that LAO has compared to the more common substrates such as sapphire and SiC. Other potential substrates with the potential of ready removal include GaAs and Si. Both of them have very poor lattice matching (>45% mismatch) to GaN. The ability to free the GaN thin film does provide great flexibility in device design and manufacture.
U.S. Pat. No. 5,917,196 to Teraguchi presents a method for growing GaN-based laser structures on LiAlO2 substrates. They report a violet laser diode emitting at 430 nm with a threshold voltage of 10V. However, they fail to disclose substrate removal so their final device may still have two contacts from above, just like with a sapphire substrate.
When dealing with an insulating substrate such as sapphire, extra steps and thus extra cost are needed to manufacture the LEDs or other devices. To reduce the LED cost, processes were developed to remove the insulating layer so that the device can be manufactured like the conventional GaAs LEDs. They include both mechanical grinding and burning with a short wavelength laser. In both cases, the removal process is very slow and not suitable for mass production. Moreover, the GaN surface after substrate removal is very rough and requires mechanical polishing or reactive ion etching (RIE) to smooth the GaN surface. With this extra effort, a new device structure is produced. This approach has been done by several laboratories and is described below.
Wong et al. have discussed the integration of a blue GaN thin film structure with dissimilar substrates by wafer bonding and lift-off (W. Wong, T. Sands, N. Cheung, etc., Compound Semiconductor Vol. 5, p. 54, 1999). They grow a nitride based device on a sapphire substrate and then use an adhesive to bond the top surface to a silicon wafer. A short wavelength laser is focused through the sapphire onto the back surface of the GaN, and a very thin film of GaN decomposes. Since Ga is a liquid and N is a gas, the sapphire falls away. By dissolving the adhesive, a nitride membrane is formed. This membrane may be transferred to another substrate. If the surface of the membrane is coated with Pd and then In, it can be flipped over and placed on a new substrate also coated with Pd. Heating melts the In, which dissolves in the Pd and forms a strong, permanent bond. A blue-emitting GaN LED has been bonded with the p-side down to a silicon substrate using this technology.
Hewlett-Packard reported transfer of a multi-quantum well nitride LED to a conducting host substrate (Y. K. Song et al, Appl. Phys. Lett., vol. 74, p. 3720, 1999). The device structure was grown by OMVPE on a standard sapphire wafer. Ni/Au contacts were deposited on the top p-type GaN:Mg layer. A copper film was then grown electrochemically on the top surface, and then the sample was flip-chip mounted onto a new host, such as silicon. After the sapphire was removed by laser ablation, a new surface contact was made to the n-type layer. Light emission from the device peaked at 450 nm.
LumiLeds Lighting reported a high-power AlGaInN flip-chip LED design (J. J. Wierer, et al, Appl. Phys. Lett., vol. 78, p. 3379, 2001). The device has a large emitting area (˜0.70 mm2) as compared with the conventional small-area (˜0.07 mm2) LEDs. The flip-chip design gives the large emitting area. Good thermal contact allows higher current and lower forward voltage and thus higher power conversion efficiencies. Around July 2002, LumiLeds introduced a 1 W Luxeon™ device using a single 1 mm×1 mm LED (425 nm with 259 mW CW at 350 mA, 3.27 V and 22.6% wall plug efficiency, Tj=25° C.), and a 5 W Luxeon™ device using 4 single 1 mm×1 mm LEDs (425 nm with 1100 mW CW at 700 mA, 7 V and 22.4% wall plug efficiency, Tj=25° C.). In their design, the sapphire substrate still covers the top of the LED. To reduce the sheet current resistance, the p-junction contact is in larger comb shape pad and the n-junction is in a finger shape. RIE (reactive ion etching) with a lithographic pattern is needed to provide the electrical connection.
Xerox reported transfer of a nitride laser to a copper substrate using laser lift-off (W. S. Wong et al, Mat. Res. Soc. Symp. Proc. V. 639, page G12.2.1, 2001). A ridge-waveguide laser structure was grown on a sapphire substrate by MOCVD. Metal contacts in the form of two micron stripes on dry-etched ridges were deposited on the top p-type surface. The structure was then flipped over and attached to a temporary silicon wafer, and then the sapphire was removed by laser ablation. After etching the new n-type GaN surface in HCl, an indium film was deposited on it. The indium was then used to bond the LD membrane onto a copper heat sink, and the temporary silicon substrate was removed.
The University of South Carolina reported using flip-chip bonding of UV-emitting GaN LED onto a silver plated copper header to obtain very high emission intensities at room temperature (A. Chitnis et al, Mat. Res. Soc. Symp. Proc. Vol. 743, p.L7.7.1, 2003), because the copper formed an effective heat-sink, and the silver provided good reflectivity for light traveling downward.
Similar lift-off techniques have been reported for GaAs-based laser structures. Bell Communications Research reported using an intermediate AlAs layer to allow wet etch removal of GaAs substrates from the LD structure (E. Yablonovitch et al, IEEE Phot. Technol. Lett., Vol. 1, p. 41 (1989)). A conventional LD was first grown by MOCVD on a GaAs substrate. Using dilute hydrofluoric acid, the GaAs substrate was removed due to the dissolution of the AlAs, allowing the epitaxial membrane to float free. The membranes, which contained a multitude of LDs, were held by wax as a support. All processing steps, including definition of the laser bars by etching as well as metallization were accomplished prior to lift-off. The structures were then mounted on new glass or silicon substrates, and the wax was removed.
Transparent substrate red AlGaInP LEDs are commercially available. Typically, Hewlett-Packard grows the LED structures on lattice-matched GaAs substrates, but the black GaAs tends to absorb roughly half of the emitted red light. Therefore, after the AlGaInP device is finished, a thick lattice mismatched GaP layer is grown over the top surface to provide a carrier. Although this top carrier is filled with structural defects, the defects do no propagate back into the active region. Then the GaAs substrate is removed by wet chemical etching. The sheet of devices is subsequently placed on a new transparent high quality GaP wafer, and sintered. Individual devices are then cut apart. It has been found that very thin films are difficult to contact and have high spreading resistance problems. Furthermore, a very thin LED chip suffers from problems with light extraction because of waveguiding and hence parasitic absorption problems at contacts and edges. Thus, the attachment of a thick transparent substrate may be very beneficial.